Gas turbine engines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. In a gas turbine engine which operates in LPP mode, fuel and air are provided to a burner chamber where they are mixed and ignited by a flame, thereby initiating combustion. The major problems associated with the combustion process in gas turbine engines, in addition to thermal efficiency and proper mixing of the fuel and the air, are associated to flame stabilization, the elimination of pulsations and noise, and the control of polluting emissions, especially nitrogen oxides (NOx), CO, UHC, smoke and particulate emissions.
In industrial gas turbine engines, which operate in LPP mode, flame temperature is reduced by an addition of more air than required for the combustion process itself. The excess air that is not reacted must be heated during combustion, and as a result flame temperature of the combustion process is reduced (below stoichiometric point) from approximately 2300K to 1800 K and below. This reduction in flame temperature is required in order to significantly reduce NOx emissions. A method shown to be most successful in reducing NOx emissions is to make combustion process so lean that the temperature of the flame is reduced below the temperature at which diatomic Nitrogen and Oxygen (N2 and O2) dissociate and recombine into NO and NO2. Swirl stabilized combustion flows are commonly used in industrial gas turbine engines to stabilize combustion by, as indicated above, developing reverse flow (Swirl Induced Recirculation Zone) about the centreline, whereby the reverse flow returns heat and free radicals back to the incoming un-burnt fuel and air mixture. The heat and free radicals from the previously reacted fuel and air are required to initiate (pyrolyze fuel and initiate chain branching process) and sustain stable combustion of the fresh un-reacted fuel and air mixture. Stable combustion in gas turbine engines requires a cyclic process of combustion producing combustion products that are transported back upstream to initiate the combustion process. A flame front is stabilised in a Shear-Layer of the Swirl Induced Recirculation Zone. Within the Shear-Layer “Local Turbulent Flame Speed of the Air/Fuel Mixture” has to be higher then “Local Air/Fuel Mixture Velocity” and as a result the Flame Front/combustion process can be stabilised.
Document WO 2009/121777 A1 discloses a lean-rich partially premixed low emissions burner for a gas turbine combustor that provides stable ignition and combustion process. This burner operates according to the principle of “supplying” heat and high concentration of free radicals from a pilot combustor exhaust to a main flame burning in a lean premixed air/fuel swirl, whereby a rapid and stable combustion of the main lean premixed flame is supported. The pilot combustor supplies heat and supplements a high concentration of free radicals directly to a forward stagnation point and a shear layer of the main swirl induced recirculation zone, where the main lean premixed flow is mixed with hot gases products of combustion provided by the pilot combustor. This allows a leaner mix and lower temperatures of the main premixed air/fuel swirl combustion that otherwise would not be self-sustaining in swirl stabilized recirculating flows during the operating conditions of the burner. The content of said document is in its entirety incorporated into this description by reference. The prior art, as disclosed in the reference, shows a burner arranged to be fueled with gas fuel.
To reduce the NOx emissions from the combustion of the main flame in a burner of the mentioned type, it is suggested to inject an emulsion of liquid fuel and water into the upstream end of the main flame and thereby decrease the temperature of the main flame. For this purpose an emulsion injection system is needed. Said emulsion could be injected by means of conventional nozzles. Examples of nozzles for similar purposes according to prior art in general are described in U.S. Pat. No. 7,568,345, U.S. Pat. No. 6,021,635 and U.S. Pat. No. 4,600,151.
Techniques of background technology are described here with references to drawings. FIG. 1 presents a drawing of a cross section of the upper half of a prior art annular nozzle. In the figure reference no. 1 represents a symmetrical axis 102 of the nozzle 101. Not gases 103 are flowing downstream along the centre of the nozzle, while inner air 104 sweeps outside an exit 105 of the hot gases 103. Said inner air passes a swirler 106 before leaving the nozzle along an outward curved wall. 107 of a first cylindrical shell 108 of the nozzle 1. Inside the cylindrical shell 108 a liquid 109 or an emulsion of two fluids flows downstream in a first annular channel 110 substantially parallel to the centre axis 102 and leaves the nozzle at an annular orifice 111. A first annular lip portion 108a and a second annular lip portion 108b of the first cylindrical shell are separated by said annular channel 110. A second outer shell 112 terminating in a second lip is arranged concentrically surrounding the first shell 108. A second annular channel 113 is formed between said first shell 108 and said second shell 112. Through this second channel 113 a flow of outer air 114 is flowing in the downstream direction and passes a second swirler 115 before the outer air 114 is discharged in parallel to the centre axis at a mouth 116 of the second channel. According to this prior art technology the inner air 104 is spread as a divergent jet, The inner air will blow the liquid, or emulsion, 109 emerging from the orifice 111 outwards, such that it meets the jet of outer air 114 emerging from mouth 116. The jet of outer air will then disintegrate the liquid/emulsion into droplets and distribute them into the jet.
By this structure of a prior art nozzle the following scheme is valid:
At the inner surfaces of the first 108a and the second 108b annular lip portions, indicated at by positions A and B: The air flow (inner air) will deviate from the surface of the nozzle body, i.e. the first shell, and thereby gives way for carbon to deposit. Some reasons for this are:                The contact surface over which fuel is flowing is cooled by the fuel (inside first annular channel 110). Due to centrifugal forces a fuel film is formed. Said surface and all metallic components of the injector are usually of equal temperature typically above 350° C. as preheat air at 350° C. for pressures at which the gas turbine engine operates heats said components. As the fuel is at low temperature, just above 15° C., at “normal” day temperature, said “cold” fuel which has high thermal capacity “cools” the surface. The surfaces at A and B will accordingly become cooled. Thereby carbon is allowed to grow at these surfaces at the same time as the cooling limits the ability to allow pyrolysis to ablate carbon away.        Initial carbon growth at said locations A and B will cause additional air flow separation and provides a chemically preferred surface on the walls at A and B for additional carbon to grow.        Carbon growth will accelerate very fast after initially being formed.        
The fuel film (or the emulsion film) is not partly generated by a tangential velocity component. Therefore:                The atomization of the liquid starts inside the first annular channel 110 already at a point where liquid is ejected.        The tangential uniformity of the liquid/emulsion film is determined by nozzle manufacturing tolerances of the gap at the annular orifice 111 formed between the two concentrical annular lip portions 108a and 108b, wherein said lip portions have a diameter much larger than the width of said annular orifice 112.        
Due to these reasons the film is not of homogenous thickness. Further:                Centrifugal force, surface tension or reduction in radius diameter (increase of swirl due to conservation of angular momentum) are the means used for achieving a unity for the film thickness.        The outer air 114 does not attack the fuel film as the film has already become separated from inner wall, which is a result of the growth of carbon deposits.        There is no place for the film to become evenly stretched by either inner or outer air flows.        A high swirl angle of inner air swirler 106 required to attempt to keep the inner air 104 attached to the walls of the first 108a and second 108b inner lip portions. A high inner swirl angle results in low inner air swirler discharge coefficient and reduces air velocity and shear acting on fuel film (while assuming that air doesn't completely separate from the wall of the lip and that a film exists). Separation of inner air is extremely likely from these reasons according to observed carbon deposits under the circumstances.        